Effects of CO2, acetylcholine and caerulein on45Ca efflux from isolated mouse pancreatic fragments
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Mouse pancreatic fragments were loaded with45Ca and placed in a flow cell. The concentration of45Ca in the effluent was measured. The effects of changing the tension of carbon dioxide on45Ca efflux were observed and compared with effects of pancreatic secretagogues.
The normal control solution was equilibrated with 5% CO2, 95% O2. Shift to solutions equilibrated with 10, 20, 50 or 100% CO2 evoked a dose-dependent increase in fractional45Ca efflux, with a just detectable effect at 10% and a maximal one at 50%.
The CO2-evoked Ca release was not due to anoxia, since a short period of exposure to a 100% N2-equilibrated solution had no effect. A decrease in extracellular pH (tris buffering) had only a very modest effect on45Ca efflux.
CO2-evoked Ca release under conditions avoiding extracellular pH changes (20% CO2, 100 mM NaHCO3). This CO2-evoked enhanced45Ca efflux was sustained during a 30 min stimulation period, but was abruptly terminated on return to the control solution (5% CO2, 25 mM NaHCO3). NH3 (10 mM) added to the 20% CO2-equilibrated solution for a brief interval in the middle of a period of CO2-evoked enhanced45Ca efflux evoked a rapid return of the fractional Ca efflux towards the resting level. This effect was rapidly reversible.
While the CO2-evoked Ca release was largely sustained, the ACh-evoked increase in45Ca fractional efflux was entirely transient. The CO2-evoked Ca release was not inhibited by a background of sustained ACh stimulation. ACh-evoked Ca release, however, was markedly inhibited in the presence of sustained CO2 stimulation.
2,4 Dinitrophenol (1 mM) in combination with iodoacetate (2 mM), while markedly reducing45Ca uptake into the fragments during the loading period had little or no effect on the ACh-evoked increase in45Ca fractional efflux. The CO2-evoked Ca release, however, was markedly reduced by these metabolic inhibitors.
The local anaesthetic procaine (1 mM) virtually abolished ACh- or caerulein-evoked Ca release without having any influence on the CO2 effect.
It is concluded that CO2 releases Ca from pancreatic acinar cells by means of intracellular acidification. This effect may in part be due to H+ displacement of Ca2+ from intracellular membrane binding sites and partly due to release of Ca from compartments (organelles) into which Ca has been actively accumulated.
Key wordsPancreas Acinar cell CO2 Intracellular acidification Ca transport Ca2+−H+ interaction ACh Caerulein
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